The properties of a polymer are of great importance in any application. For biomedical polymers, the most important single property is probably biocompatibility, which refers to the interactions of living body tissues, compounds and fluids including blood with any implanted or contacting polymeric material. Each system of polymer-body tissue interactions must be studied individually in terms of physical properties including tensile strength, modulus (flexibility) and abrasion resistance, polymer stability, general tissue-fluid interactions and blood compatibility.
Polyurethane block copolymers have been proposed for use in blood-contacting applications because of their generally excellent physical properties and relatively good blood compatibility. Lelah and Cooper, Polyurethanes in Medicine, CRC Press, Boca Raton, Fla. (1986). Thermoplastic polyurethane block copolymers of the (AB).sub.n type have alternating soft and hard segments. In conventional segmented linear polyurethanes, the soft segments are often low molecular weight (600-3000) polyether or polyester macroglycols, and the hard segments usually comprise an aromatic diisocyanate that has been chain extended with a short chain diol.
It is desirable to further improve the blood compatibility of these materials to allow their use in such demanding applications as small-diameter vascular grafts, catheters, kidney dialyzers, cardiac assist devices and the artificial heart.
A segmented polyurethane can exhibit a wide range of physical properties and morphologies depending on the chemical structure of the soft and hard segments. The soft segments provide flexibility, whereas the hard segments are more rigid and provide tensile strength and wear or abrasion resistance. In general, segmented polyurethanes demonstrate excellent mechanical properties which are directly related to their two phase morphology.
Polysiloxane-based elastomers, and in particular polydimethylsiloxane-based materials, demonstrate desirable characteristics including extremely low glass transition temperatures (T.sub.g), good thermal and oxidative stabilities, low surface energies and good electric properties. Because of their good biocompatibility and low toxicity, polydimethylsiloxanes have been used in biomedical applications. A. Braley, J. Macromol. Sci., Chem., A4, 529 (1970) and Ward, Jr. et al., Organometallic Polymers, Academic Press, New York (1978).
Low molecular weight polydimethylsiloxanes (PDMS) have been incorporated as soft segments in polyurethane block copolymers. Yu et al., J. Polym. Sci., Polym. Phys. Ed., 23, 2319 (1985) and G. L. Gains, Macromolecules, 14 208 (1981). As a result of the large difference in the solubility parameters of soft and hard segments, however, these polydimethylsiloxane-based polyurethanes are likely to be highly phase-separated materials resulting in rather poor mechanical properties. Yu et al., id. and Speckhard et al., Rubber Chem. and Tech., 59, 405 (1986). Premature phase separation may also occur during synthesis of these materials to produce compositional heterogeneity and low overall molecular weight. Speckhard et al., id.
In accordance with several studies concerning the physical properties of polydimethylsiloxane-based polyurethane block copolymers (Speckhard et al., id. and Yu et al., id.), increases in the degree of phase mixing and improvement in the hard domain cohesion can lead to enhanced physical properties. For example, improved tensile properties for poly(chloropropylmethylsiloxane) polyurethanes and poly(cyanoethylmethylsiloxane) polyurethanes have been obtained following the introduction of polar side groups along the polydimethylsiloxane soft segment backbone [Yu et al., id. and Li et al., id.]. These polar side groups promote phase mixing and improve interfacial adhesion which are important factors in determining the mechanical properties of the two-phase materials.
A continuing need exists for improved polymeric materials that are suitable for blood-contacting applications and which exhibit the necessary tensile properties and abrasion resistance for long-term use.